Systems that control electric lighting in response to available daylight must be calibrated for proper operation. This process generally involves derivation of a “daylight ratio” that predicts the light contributed to the occupant area as a fraction of that seen by a light sensor, and also a measure of the total illumination contributed by the available electric lights. To measure the latter quantity, the electric light sources must be “warmed up” for some period of time, so that they have attained their full intensity prior to measurement. Prior-art approaches generally have used a predetermined time delay (e.g., two minutes), and assumed that the light sources have reached their full intensity by this time. Unfortunately, such an approach is flawed, as the warm-up time to reach full intensity is a function of both the type of lamp (fluorescent, LED, incandescent, HID, etc.) and also can be dramatically affected by the air temperature. Furthermore, impatient installers will sometimes attempt to shorten this time, resulting in inaccurate calibrations.
While light sensing controllers that view only the daylight or natural light sources have been employed for many years, they are typically difficult to commission, requiring both a special tool, a calibrated light meter, and appropriate training.
Because the light output of fixtures declines over time, it is necessary to periodically relamp the fixtures to maintain an acceptable illumination intensity in the task area. The general practice has been to simply replace lamps on a calendar schedule, because it has been difficult to determine when the relamping is truly required. Because different types of lamps degrade at different rates, and the rates may also be affected by environmental conditions, this is a suboptimal strategy. It is also understood by those skilled in the art that the output of most light sources such as fluorescent lamps, LEDs, and others, declines over time, with age. Conversely, when lamps are replaced with new ones, the light output will generally increase. To continue to accurately compensate for daylight changes, over time, it is necessary for the sensor to learn the changes in the available electric light, and update its setting for maximum electric light available in the space, “max EL”. This process is sometimes referred to as “recommissioning” and conventionally requires skilled installers.
For accuracy, it is preferable to conduct the recommissioning at night, when the daylight contribution is negligible. When an electric light sensor is present in the design, prior art teaches entering the recommissioning state when the electric light sensor measurement drops below a threshold that would indicate that the sun has set. This threshold is generally a fixed, predetermined value, or perhaps one that can be manually adjusted. Recommissioning the sensor, to determine the present maximum electric light value, requires turning on all controlled loads to their maximum intensity, allowing them to warm-up to maximum light output, and then measuring the resultant illumination intensity. This process will generally take a number of minutes (more than two minutes, in general), and this action may annoy or distract building occupants.
Unfortunately, experience shows that the light threshold required to recommission a system will vary from installation to installation, due to potential stray light that may enter into the light sensor, either through reflectance of electric light from the interior surface of a skylight, or through external light pollution entering through a skylight at night.